chrome-devtools-frontend/.agents/skills/devtools-verification/SKILL.md

Chrome DevTools UI

---
name: devtools-verification
description: "MANDATORY: Activate this skill ANY TIME you need to build the project, run tests, or verify code health in DevTools. You MUST use this skill before executing commands like npm test, npm run build, autoninja, or linters, as it contains critical, repository-specific instructions on how to correctly format these commands, filter test runs, and interpret failures."
---

# Instructions on how to verify your changes

## Testing

- To test a file, you can run `npm run test -- <FILEPATH>` where `FILEPATH` is a path to a `*.test.ts` file, relative to the working directory.
- Test files are usually defined alongside their implementation. So if you are working on `front_end/panels/timeline/TimelinePanel.ts`, you would expect the test file to be defined in `front_end/panels/timeline/TimelinePanel.test.ts`.
- You can also test an entire directory. For example, `npm run test -- front_end/models/trace` will run all tests in that directory.

## Building & compiling

- Check for build issues by running `autoninja -C out/Default`.

## Linting

- `npm run lint` will execute ESLint and StyleLint. It will report any violations and automatically fix them where possible.
- To run the linter on a specific file or directory, you can run `npm run lint -- <PATH>` where `PATH` is a path to a file or directory. This will also automatically fix violations where possible.

## Fetching Tryjob Results, Failure Logs, & Screenshot Goldens

Use `scripts/tools/get_cl_test_results.py` to fetch all tryjob build statuses, per-test failure logs, Blink web test text diffs, and screenshot artifacts (`actual_image`, `expected_image`, `image_diff`) for a CL in one command:

```bash
# Fetch all build statuses, failure logs, and artifacts for a CL into an output directory:
scripts/tools/get_cl_test_results.py --cl <CL_NUMBER> --output-dir <OUTPUT_DIR>

# Filter for specific failing tests (e.g. DOMTreeWidget) or specific builders:
scripts/tools/get_cl_test_results.py --cl <CL_NUMBER> --test-filter "DOMTreeWidget" --output-dir <OUTPUT_DIR>

# Also apply Linux actual_image screenshot artifacts directly to test/goldens/linux/...:
scripts/tools/get_cl_test_results.py --cl <CL_NUMBER> --output-dir <OUTPUT_DIR> --apply-goldens
```

### Screenshot Golden & Inspection Rules
- **Linux-only goldens**: Never generate, update, or commit golden PNGs from macOS. Screenshot tests in DevTools only use Linux goldens (`test/goldens/linux/...`) produced by Linux trybots (`dtf_linux_rel` / `dtf_linux_dbg`). Do not run screenshot tests locally on macOS to verify goldens.
- **Multimodal inspection only**: Always inspect `actual_image.png`, `expected_image.png`, and `image_diff.png` directly using your multimodal file viewer (`view_file`). Never write or run programmatic image comparison scripts.

## Best practices

- Run tests often to verify your changes.
- Prefer using a fast build, if it exists, to keep the feedback loop shorter.
- Periodically build to check for errors.
- Run `git cl presubmit -u` at the end of your code changes.